Deltas on the move; Making deltas cope with the effects of climate c

KvR report 001/2006almost completely flooded during flood seasons, and some nutrients and silt reachedthe flood plain. This regular flooding had a major impact in the following ways:- making the flood plain soil naturally fertile;- desalinization of the flood plain;- refreshing the water in the lakes;- creation of natural biotopes for plants and animals.However, the capacity of the deep main river channel was so large that most of thewater, along with most of the silt and nutrients and all of the sand, was transporteddirectly to the sea. The Romanian part of the delta is still regularly flooded duringspring floods, but the Ukrainian, northern part of the delta receives almost no water,sediments or nutrients since the construction of embankments over the last 40 years(Figure 7.1, WWF, 2002). This has significant consequences for several keyprocesses in the delta as a whole since 75% of the water flows through the northernpart of the delta:- Slow growth of the inner parts of the delta. As much of the sediment passedthrough the delta to the sea, there was not much available for building up theheight of the delta. These circumstances account for the existence of the veryextensive plains with reed beds and lakes in the delta. The embankments built inrecent time, sectioning off large parts of the Ukrainian flood plain and most of theislands in the northern distributary, have completely halted the growth process.- Filtering of the river water. When the waters of the Danube reached the immensereed beds and lakes of the vast flood plain over a four-month period each year, atleast some of the water was filtered by the flood plain: silt particles were removedand nutrients supported the luxuriant reed growth. No filtering has taken place inthe reed beds since the construction of embankments.- Formation of river banks. During flooding, silt particles in the water weredeposited on the riverbanks and in the adjacent stretches of reed beds. In thepresent embanked situation, there is virtually no sedimentation except for thenarrow strips of land not encircled by dikes.- Development of islands. Islands in the river developed in the same way as theriverbanks but in a distinctive form. This is the reason for their saucer-like shape,high at the outer edges and lower in the middle. Nowadays the islands areembanked, sedimentation is absent and the center of the islands subsides due toconsolidation;- Growing in line with the sea. The water level in the oceans and seas has risen byabout 30 cm over the last two centuries. The siltation rate of the Danube floodplain has apparently kept pace with the rising water level in the Black Sea,despite being quite slow. The riverbanks remained dry, while insufficient sedimentreached the main area of the delta plain, the extensive reed beds and the shallowlakes to keep them completely dry, although they were not flooded by the seaeither.It is predicted that the sea level will rise at a faster rate in the near future, at anestimated 50–90 cm this century alone. In order not to become completelysubmerged, the area must be accessible for the available sediment. This shouldallow it to keep pace with the sea level.The outer deltaThe quantity of sediment that reaches the outer delta, which lays completely inUkraine, is very important for the growth of this most dynamic part of the deltasystem. The quantity of sediment increased during the 19th century when a larger72<strong>Deltas</strong> on the movearea of land in the catchment area was brought into cultivation. On the other hand,the influx of sediment decreased sharply after the construction of the Iron Gate damsin the mid-20th century. The isolation of the flood plain from the (rest of) the delta bythe embankments further increased the influx of sediment.A number of important processes take place in the outer delta:- Formation of sandbars. The sediment carried by the river to the sea is subjectedto the forces of the waves and currents, along with sand originating from the sea.The force of the waves creates sandbars in the coastal area. These sandbarsdevelop parallel to the coastline in a north–south direction. At the same time, thesea currents transport the sand southwards, hence the asymmetric fan-like formof the outer delta. Locally, the sandbars develop into dunes.- Formation of numerous outlets, reed beds and the filtering of silt and nutrients.The sandbars block the river mouths at the point where the river meets the sea.Behind the sandbars, the dunes, and the shallow river mouths, the waterstagnates. The discharge of the Kiliya branch flows through many smalldistributaries towards the sea, and seeps through the reed beds in places. Theouter delta therefore has a bowl-like shape: a series of ever-renewing sandbarsat the outer edges of the delta with behind them an area where silt is deposited,reed beds develop, and many branches bifurcate.Neither dikes nor embankments have been constructed on the outer delta, so thearea has retained many of its natural qualities. However, some of the larger riverbranches used to be dredged for shipping, providing an easy artificial outlet for thewater, sediment and nutrients. Both the filtering capacity and the growth of the deltahave been negatively affected by the dredging.Potential for a natural systemIn order to restore the ecology and functioning of the delta, a choice needs to bemade as to whether the problems in the area should be solved by means oftechnology or by making use of the natural processes as far as possible. The firstoption involves numerous technical solutions for isolated problems. Some problems,however, would be too large for technical solutions; for example, the filtration of thewater of the Danube, sedimentation on riverbanks and in the reed beds at a rate thatwould allow the delta as a whole to grow in line with the expected rise in the sealevel.The second method implies the restoration of the natural system as a whole. The reestablishmentof the natural morphological system by reconnecting the flood plainand the islands to the river by removing the dikes (as far as possible) will solve manyof the problems with which the delta flood plain is now confronted. Due to theembankments, almost the entire natural system of the flood plain is currently in astate of hibernation. Restoring the annual flooding would bring most of the naturalprocesses to life again.A transition zone from river to sea, from freshwater to salt water, and from clay tosand can be found near the sea. This part of the delta has proven its ability tomaintain its rich and healthy state to date. It is continually changing in time andspace, and will follow the major climatic changes of our era, i.e. the rising of thewater level in the Black Sea.73